100-C 25-C 50-C 75-C C+M 50-C+M C+Y 50-C+Y M+Y 50-M+Y 100-M 25-M 50-M 75-M 100-Y 25-Y 50-Y 75-Y 100-K 25-K 25-19-19 50-K50-40-40 75-K 75-64-64CHAPTER 1
The Solar System and Its
Place in the Galaxy
Paul R. Weissman
Jet Propulsion Laboratory
California Institute of Technology
Pasadena, California- Introduction 5. The Solar System’s Place in the Galaxy
- The Definition of a Planet 6. The Fate of the Solar System
- The Architecture of the Solar System 7. Concluding Remarks
- The Origin of the Solar System Bibliography
1. Introduction
The origins of modern astronomy lie with the study of our
solar system. When ancient humans first gazed at the skies,
they recognized the same patterns of fixed stars rotating
over their heads each night. They identified these fixed pat-
terns, now called constellations, with familiar objects or an-
imals, or stories from their mythologies and their culture.
But along with the fixed stars, there were a few bright points
of light that moved each night, slowly following similar paths
through a belt of constellations around the sky (the Sun and
Moon also appeared to move through the same belt of con-
stellations). These wandering objects were theplanetsof
our solar system. Indeed, the name “planet” derives from
the Latinplaneta, meaning wanderer.
The ancients recognized five planets that they could see
with their naked eyes. We now know that the solar system
consists of eight planets, at least threedwarf planets, plus
a myriad of smaller objects: satellites, asteroids, comets,
rings, and dust. Discoveries of new objects and new classes
of objects are continuing even today. Thus, our view of the
solar system is constantly changing and evolving as new data
and new theories to explain (or anticipate) the data become
available.
The solar system we see today is the result of the complex
interaction of physical, chemical, and dynamical processes
that have shaped the planets and other bodies. By studying
each of the planets and other bodies individually as well
as collectively, we seek to gain an understanding of those
processes and the steps that led to the current solar system.
Many of those processes operated most intensely early in
the solar system’s history, as the Sun and planets formed
from an interstellar cloud of dust and gas, 4.56 billion years
ago. The first billion years of the solar system’s history was
a violent period as the planets cleared their orbital zones
of much of the leftover debris from the process of planet
formation, flinging small bodies into planet-crossing, and
often planet-impacting,orbitsor out to interstellar space.
In comparison, the present-day solar system is a much qui-
eter place, though many of these processes continue today
on a lesser scale.
Our knowledge of the solar system has exploded in the
past four decades as interplanetary exploration spacecraft
have provided close-up views of all of the planets, as well as
of a diverse collection of satellites, asteroids, and comets.
Earth-orbiting telescopes have provided an unprecedented
view of the solar system, often at wavelengths not acces-
sible from the Earth’s surface. Ground-based observations
have also continued to produce exciting new discoveries
through the application of a variety of new technologies such
as charge-coupled device (CCD) cameras, infrared detec-
tor arrays, adaptive optics, and powerful planetary radars.
Theoretical studies have also contributed significantly to
our understanding of the solar system, largely through the